The model is based on the concept of tissue homeostasis, defined by physiotherapists as ‘active maintenance of constant conditions in the internal environment’.

All living structures are comprised of cells that normally are metabolically active, with continuous physiological maintenance of tissue at the molecular level. A single loading event of sufficient magnitude, or a series of repetitive loading events of lesser magnitude can cause an injury inducing a cascade of reparative biochemical processes which reflect a loss, at least temporarily of normal tissue homeostasis.

The article is linked above and I strongly suggest the reader checks it out as there is a series of diagrams that illustrate how the envelope looks graphically in terms of load specific activities, or alternatively see how I draw it for patients [HERE / layman luke icon]

Scott Dye coined the term ‘envelope of function’ and described it as ‘That torque which can be safely withstood and transmitted by the system without damage’, the torque envelope of that system. A human knee is a very different system to a car, but both can be described as able to accept, transfer and dissipate respective forces, mechanical in the car, biomechanical in the knee, both describing its envelope of function.

“A knee is a very different system to a car, both however can be described in terms of their ability to accept, transfer and dissipate forces, one being mechanical, the other biomechanical, this is its envelope of function”

There are a series of zones that are described in the tissue homeostasis model that are worth introducing as themes here, though as stated, graphical representation is key to understanding and especially implementing clinically.

Envelop of Function (EoF)

Most uninjured joints can withstand a range of 1-8 times body weight and maintain homeostasis; this range is termed homeostatic loading, the outer range of which is the EoF.

Zone of Homeostasis (ZoH)

This is the region underneath the EoF and is the range of loading across any given joint compatible with and inductive of maintenance of tissue homeostasis.

Zone of Subphysiological Underload (ZoSU)

This is a region beneath the ZoH where prolonged bed rest, immobilisation or tissue under loading (such as space travel) induce a loss of tissue homeostasis and can manifest as muscle atrophy and calcium loss from bones secondary to disuse can ensue.

Zone of Supraphysiologic Overload (ZoSO)

This region of increased load not sufficient to cause immediate overt damage is the zone of supraphysiological overload and is just beyond the EoF. A more acute form of supraphysiological overload may include a dashboard injury for example. Careful manipulation of any well designed training program works in this zone.

Zone of Structural Failure (ZoSF)

If even greater load than that in the ZoSO is applied then overt structural damage ensues, take for example an anterior-cruciate-ligament (ACL) rupture, immediate injury.

What this heuristic model enables us to do clinically is take a thorough history of all the activities that may be playing a role in the genesis of symptoms. The model is a dynamic one where the EoF shifts, evolves and can be manipulated over time – the latter being the very goal of athletic endeavour, expanding ones envelope. If we have an accurate set of activities and current provocative movements or tasks we can place them either above or below their current EoF. This process also serves as a valuable needs analysis[link here to article about successful outcomes for patients compared to FMS eg) into what is meaningful for that person and where rehabilitation needs to take them.

What Scott Dye then discusses is rigorous restriction of those activities to find their pain free capacity. Their ZoH has theoretically shrunk in such a way that activities previously beneath the EoF are now above it and triggering pain, walking down steep steps for example.

A clear understanding of what pain is and isn’t is critical as application to chronic pain is a limitation of the tissue homeostasis model, hurt does not equal harm in these instances, there is not necessarily a homeostatic issue in the tissue. Neuroscience researcher and physical therapist Adrian Louw’s ‘alarm system’ [link article here] analogy serves as a very powerful way to communicate when we have nudged into the EoF and what this means to the patient. If the presentation is chronic, then clinical reasoning and management should lean more towards de-sensitization of the nervous system. Graded exposure and incremental progression over time are still pertinent in these cases, however outcomes are more clinically gauged by function than pain.

“Chronic pain is an exception here, hurt does not equal harm in this instance, there is not necessarily a homeostatic issue in the tissue”

It is important to state here that while the authors rigorously restrict those activities triggering the alarm, they are NOT advocating a sedentary being. The message simply has to be clear that we are finding where their current EoF is and then remain as active as possible at their upper threshold of their ZoH.

T-shirt reads 'life begins at the edge of your comfort zone'. One might swap comfort zone for 'Envelop of Function'!!

The concept of expanding ones EoF over time from a dynamic starting point, found through trial and error really is the hallmark of graded exposure [link article of graded exposure] and activity, small incremental progression over time. It is not unusual for a runner to suddenly transition to a higher volume of training without the necessary adaptation it requires. Their envelope of function is not large enough for that task and medial-tibial-stress-syndrome (shin splints) is just round the corner.

The framework enables a clinician to therapeutically transition from clinical rehabilitation to preventative conditioning work or put another way; training. The quicker we can get people to the training end of the spectrum the quicker we can build self efficacy in those we work with, the quicker we can build robust capable people and is ultimately my end goal [link loading continuum]. It is also my opinion that this is where the real fun stuff happens, why hand that fun over to someone else!!! (Personal trainer, Strength & Conditioning).

{ insert video exaggerating error drill with a game}

The versatility of the model enables it to be applied as a principle as oppose to a technique. Give a man a fish and he’ll eat for a day, teach him HOW to fish and he’ll eat for life. How this model is applied is only limited by the application and intuition of us as clinicians as it has far reaching implications.

An example discussed in my mentorship was how this applies to not only tissue loading capacity, but equally to sun exposure. Incremental, small graded exposure of sun to pale skin over time is going to slowly nudge the metaphorical EoF into the ZoSO; expanding ones’ ZoH such that the skin will develop tolerance, darken and be able to withstand longer sun exposure. Contrast that to a pale person who doesn't go through the adaptation described and exposes to long spells in the sun without expanding their ZoH and they will get significant sun burn, or structural failure / injury.

“Give a man a fish and he’ll eat for a day, teach him HOW to fish and he’ll eat for a lifetime”

— Chinese Proverb

Some closing thoughts

The tissue homeostasis model is not a one size fits all, cook-book framework, but a principle that enables clinicians to regress and progress activities / exercises (or sun tanning!) as part of a thorough Bio-Psycho-Social approach. Perhaps more crucially, and what really satisfies a lot of my own movement and preventative biases, it enables a framework that can bridge clinical management and conditioning, where the real magic happens.